1. Field of Invention
This invention relates to electrolytes and electrochemical storage devices. More particularly, this invention relates to high performance nonaqueous electrolytes and electric double layer capacitors (supercapacitors).
2. Description of Related Art
An electric double-layer capacitor (EDLC) is an electrochemical capacitor that has an unusually high energy density compared to common capacitors. They are of particular interest in applications requiring high rates of charge and discharge, little degradation of potential over hundreds of thousands of cycles, good reversibility, and high cycle efficiency.
Typically, EDLCs are constructed with nonaqueous electrolytic solutions containing quaternary ammonium in one or more organic solvents such as propylene carbonate, gamma-butyrolactone, acetonitrile, sulfolane derivate, and linear carbonates such as N-diethyl methyl chitosan (DEMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC). Although an EDLC can utilize aqueous electrolytes, such as those containing mineral acids, it is known that the maximum operational voltage of such EDLCs is typically limited to 0.8V because water in the electrolyte undergoes decomposition above this voltage. In contrast, EDLCs using anhydrous nonaqueous electrolytes can operate at a voltage of over 2.5V. EDLCs using nonaqueous electrolytes also are capable of storing, in certain cases, up to 10 times more energy than typical EDLCs using aqueous electrolytes because the electrostatic energy content of a EDLC corresponds to the square of the maximum operation voltage.
However, it is also known that EDLCs which utilize non-aqueous electrolytes suffer from an internal pressure buildup as they age. This pressure buildup can result in the EDLC's seals leaking or rupturing. To mitigate this pressure buildup, EDLCs are often designed to operate at voltage below 2.5V.
High operating voltages also tend to lower the lifetime of an EDLC due to pressure buildup. The pressure increase inside the cell depends on the electrode characteristics and on the electrolyte characteristics. For example, EDLCs with nonaqueous electrolytes operate well above the decomposition voltage of water and, therefore, any water impurities initially present in the cell, particularly water contained in the electrolyte, will quickly undergo electrolysis resulting in gas formation. This gas formation leads to a quick pressure buildup during the first charging. Accordingly, it is common to use a nonaqueous electrolyte solution that is as dehydrated as possible.
Interactions between the electrode and the electrolyte can also lead to gas generation. The positive and negative electrodes of an EDLC commonly are constructed of activated carbon or graphite. These materials typically contain functional group impurities, such as hydroxyl, carboxyl, carbonyl, and/or ketone moieties, which undergo electrochemical redox reactions upon application of a voltage, thus leading to gas generation. Therefore, the activated carbon material for an electrode is carefully selected to minimize functional group impurities.
Although problems associated with gas build up are usually detected by careful lifetime tests, in which the capacitors are subjected to the nominal voltage at elevated temperatures for a representative period, this long lasting test yields little information about the origin of the underlying chemical reactions leading to the failure. Beyond the use of carbon with less functional groups and well dehydrated nonaqueous electrolyte solutions, scant information is available regarding the mechanisms responsible for pressure build-up in an EDLC or means for its reduction.
Although little is known about the conditions that effect the pressure build-up in EDLCs using anhydrous nonaqueous electrolytes, others have addressed the problem of pressure build-up in lithium ion batteries. For example, Salmon et al. (U.S. Pat. No. 5,378,445) describes the use of ammonia gas to treat a lithium ion battery electrolyte to eliminate gas generation due to the presence of acid. Zhongyi Deng et al. (US 2006/0269844) addresses the gassing problem in lithium ion battery electrolyte caused by the presence of acid by treating the electrolyte with triazine compounds.
Although the lithium ion battery typically uses a nonaqueous electrolyte, the composition of the salt, as well as the solvent, is quite different than super capacitors. As a result, the operational characteristics of lithium ion batteries are not directly applicable to the types of nonaqueous electrolyte used in electric double-layer capacitors. In particular, lithium ion batteries use nonaqueous electrolytes comprising lithium hexafluorophosphate (LiPF6) salt, which exhibits very good electrochemical stability and conductivity when dissolved in binary or ternary solvents including cyclic carbonates such as ethylene carbonate (EC) (also called cyclic ethylene ester), and linear carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC). However, LiPF6 is not thermally stable and readily decomposes at elevated temperatures forming gaseous and very reactive PF5. Under such conditions PF5, in turn, typically reacts with solvents or LiPF6 to produce more gaseous compounds.
EDLC electrolytes avoid this problem because they do no contain the lithium hexafluorophosphate (LiPF6) salt. Also, unlike lithium ion batteries, EDLC have electrodes that are constructed, for example, of activated carbon. The activated carbon is produced by partial oxidation of carbon using steam, acid or alkali, and thus invariably possess functional groups like hydroxyl, carboxyl, ketone, and carbonyl which participate in a redox reaction to form carbon monoxide, carbon dioxide, methane and hydrogen. EDLC electrolytes can also utilize cyclic carbonate and nitrile solvents and tetraalkylammonium tetrafluoroborates.
Honda, et al., (US 2003/0202316) reports the additives chlorobenzene and fluorobenzene can be used to reduce pressure build up in EDLCs due to gas generation. However, it is believed that these additives work via a physical mechanism by blocking the access to or covering the catalytic surfaces in the carbon.
Thus, there remains a need for methods for reducing the pressure inside an EDLC and for EDLC's having long life and/or a high operating voltage.